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Abstract:

A method and a network node are provided, for distributing timing
information in a packet-switched network. The method is characterized in
that at least one network node used as a quasi slave node is located at a
path extending between a master node and a slave node. The method
comprising a step of calculating the timing at that network node, by
passively processing packets belonging to a PTP-type packet stream being
exchanged between the master node and the slave node and conveyed via
that intermediate network node operating as a quasi slave.

Claims:

1. A method for timing distribution in a packet-switched network, said
method being characterized in that at least one network node used as a
quasi slave node is located at a path extending between a master node and
a slave node, wherein said method comprising a step of retrieving timing
at said network node by passively processing packets belonging to a
PTP-type packet stream being exchanged between a master node and a slave
node and conveyed via said network node operating as a quasi slave node.

2. The method according to claim 1, wherein the packet-switched network
comprises an Ethernet domain, and said PTP-type packet stream is a
PTP-type packet stream which is transmitted in conformity with IEEE
Recommendation 1588v2.

3. The method according to claim 1, wherein the slave node is operative
to exchanging PTP-type messages with the master node to enable obtaining
the slave node's timing with reference to the timing of the master node,
said method comprising: selecting an intermediate network node located at
said path between the master and slave nodes to serve as a quasi slave
node; ensuring that the intermediate network node has at least partial
functionality of the slave node; at the intermediate network node,
passively processing said PTP-type stream and retrieving the timing for
said quasi slave node; if the timing retrieved by said quasi slave node
does not match one or more predetermined criteria, generating an
indication that relates to this mismatch.

4. The method according to claim 1, wherein the passive processing of the
PTP-type stream at said network node comprises generating internal
timestamps for said network node, and associating said internal
timestamps to packets of said PTP-type stream arriving at said network
node.

5. The method according to claim 1, wherein the processing includes the
steps of clock & phase recovery, measuring packet delay and/or packet
delay variation PDV for said network node.

6. The method according to claim 3, wherein the predetermined criteria
comprise a specific value of time delay and/or PDV at the point where
said network node is located.

7. The method according to claim 1, wherein a plurality of network nodes
used as a quasi slave node are located along a path extending between the
master node and the slave node.

8. A network node adapted to operate in a packet-switched network and
provided with hardware and/or software means enabling the network node to
operate as a quasi slave node, adapted to intercept and passively process
packets belonging to a PTP-type packet stream being exchanged between a
master node and a slave node and conveyed via said network node operating
as an intermediate node, and retrieve timing information therefrom.

9. The network node according to claim 8, wherein the packet-switched
network comprises an Ethernet domain and said PTP-type packet stream is a
PTP-type packet stream which is transmitted in conformity with IEEE
Recommendation 1588v2.

10. The network node according to claim 8 and comprising a processing
unit adapted to process the packets belonging to a PTP-type packet
stream, said processing unit is adapted to process said packets and
retrieve timing for said network node, and in case the timing retrieved
by said network node does not match to one or more predetermined
criteria, said processing unit is further adapted to generate an
indication related to this mismatch.

11. The network node according to claim 8, adapted to operate selectively
in a conventional slave mode and/or in a quasi slave mode.

12. The network node according to claim 8, operative to be installed as
an edge node located at the border of an Ethernet network domain and of
an access network, thereby enabling carrying out diagnostics of problems
in the Ethernet network domain and/or said access network.

13. A computer readable medium storing a computer program for performing
a set of instructions to be executed by one or more computer processors,
the computer program is adapted to perform the method of claim 1.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of Israel Patent Application No.
217232, filed Dec. 27, 2011, the disclosure of which is incorporated by
reference herein in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of monitoring and
managing communication networks, more specifically to monitoring specific
parameters which are important for controlling timing distribution over
packet-switched networks.

BACKGROUND OF THE INVENTION

[0003] In the following description the following abbreviations will be
used:

[0004] MEN--Metro Ethernet Network

[0005] PTP--Precision Timing
Protocol (such as described in IEEE 1588v2)

[0006] PE--Provider Edge

[0007] PDV--Packet Delay Variation

[0008] CPE--Customer Premised
Equipment

[0009] eNodeB--Base station of LTE (Long term evolution 4G
networks). Master node and Slave node--network nodes establishing
there-between a packet-message stream, for example a PTP stream, mainly
for providing the Slave node with timing in reference to the Master node.
FIG. 1 (prior art) shows one option of monitoring/measuring parameters of
data transmission via a packet-switched network 10, such as MEN. Let us
assume that we are interested in providing so-called timing distribution
in the network. FIG. 1 illustrates a setup for measuring PDV for an
exemplary Ethernet Service being transmitted through a provider network
such as MEN 10, between two demarcation points located at two peer
elements PE: PE(source) 12 and PE-C 14 (destination, customer side). Peer
element 12 may be (or be connected to) a master node such as a node
setting time clock which is considered absolute for the network, while
peer element 14 may be (or be connected to) a slave node obtaining its
time clock with reference to the clock of the master. In other words,
timing distribution is being provided throughout network 10. The timing
distribution is closely related to monitoring/measuring of packet delays
or packet delay variation PDV at slave nodes (say, node 14), with
reference to the master node. To measure delays of the Ethernet service
in the packet-switched network 10, one may create an Ethernet CFM
(Connectivity Fault Management) protocol packet bidirectional stream 11
between the two peers PEs 12 and 14 (e.g., CFM with Y.1731 delay
measurement extension of the CFM).

[0010] One drawback of such a solution is that, though being sensitive to
PDV, the CFM packet stream is different from a PTP protocol pattern. The
PTP (Precision Time Protocol) is widely accepted in modern networks and
is therefore preferred by the majority of service providers and
operators.

[0011] Another drawback is that the CFM stream 11 consumes a certain
amount of bandwidth BW in the MEN.

[0012] As already mentioned, in order to distribute timing (frequency and
phase) information in a modern data packet-switched network, it is
preferred to use the PTP protocol over the packet-switched network.

[0013] FIG. 2 (prior art) illustrates an example of a setup in which a PTP
protocol is used for such a purpose in a data packet-switched network
comprising MEN 10.

[0014] The term "timing distribution" as used herein should be understood
for example (but not exclusively), as enabling a time schedule in a
cellular network for interaction between cellular base stations in order
to regulate time slots of their communication with the users mobile
devices.

[0015] The PTP protocol is suitable for the above purposes and is thus
sensitive to PDV distribution in the network. Therefore it would be
reasonable to monitor/measure PDV for a PTP service, e.g. along a network
path of the service. Assuming that there is a number of cellular base
stations that form a chain in the network, it is accepted in the art that
each of these base stations receives its own PTP service from a central
(master) node, and monitors the timing distribution (for example,
measures PDV) at the site of the specific base station for its specific
PTP service. In case the PDV for the PTP service exceeds a desired
predetermined value, the operator of the network may wish to receive an
indication (alarm) that such a condition had occurred.

[0016] The IEEE 1588v2 standard provides a conventional way of monitoring
the timing distribution (with measuring delays/PDV), by creating
bidirectional data flows specifically dedicated for the purpose of timing
distribution. The functionality of timing distribution is conventionally
performed by specifically equipped network elements, and such
elements--for example, cellular base stations NodeB--usually perform the
full set of the required operations (clock+phase recovery, the discussed
delay measurement, etc.).

[0017] In FIG. 2, the standardized PTP technique orders to launch a PTP
data stream 1 between the PTP master node 16 and a first customer node
PE-C1 (Slave 1, marked 14). Since slave 1 is connected in chain with
Slave 2, a PTP data stream 2 is established between master node 16 and
PTP Slave node 2, 20. At least one of the slaves may be implemented in a
Customer's Premised Equipment ("CPE") such as CPE of NodeB. Each of the
PTP Slaves 1 and 2 should be able to recover the clock and the phase of
the received packets, and to perform necessary PDV
monitoring/measurements for the specific respective data streams.

[0018] Any additional node (such as node 18 or node 22) in the network
would also need timing distribution with reference to the master node 16.
It would therefore need to be provided with an additional PTP slave
functionality in each of the nodes (Slave 3, Slave 4), and would require
establishing an additional PTP service flow for each of the slave nodes
(see PTP stream 3 and PTP stream 4).

[0019] One of the main drawbacks of the conventional setup illustrated in
FIG. 2 is in that the PTP master 16 has to handle an additional Slave
function for every additional Slave node, i.e. to create an additional
PTP service stream, a task which is not always possible due to a limited
number of Slave functions which may be supported by a single PTP Master.
Also, every new Master-Slave stream also consumes a certain amount of BW
in the MEN.

OBJECTS AND SUMMARY OF THE DISCLOSURE

[0020] It is therefore an object of the present invention to provide a
method which would overcome the drawbacks of the conventional known
methods for timing distribution in modern packet-switched networks
(comprising domains such as Metro Ethernet networks MEN, MPLS, pure
Ethernet, Ethernet Provider Bridge, IP/UDP, Ethernet over MPLS, etc.)
while using PTP-type protocol. The operation of timing distribution is
widely known, especially for cellular networks and usually comprises
processing of timing specific for a particular node (so-called slave
node) in the network, the processing being accompanied with clock & phase
recovery, measuring packet delay and/or packet delay variation PDV, and
the like for that particular node.

[0021] In one preferred example of the invention, there is provided a
method for carrying out various necessary measurements in order to enable
timing distribution in a packet-switched network, by providing one or
more virtual/snooping/quasi slave nodes capable of calculating parameters
(such as timing), at certain points of the network without producing
additional traffic and/or signaling, just by passively utilizing
(processing) PTP-type packet streams already existing in the network.

[0022] The PTP-type packet stream should be understood as being a service
packet stream with a possibility of having a time-stamp associated with
its packets.

[0023] For the sake of simplicity, we will further describe the invention,
assuming that the packet-switched network comprises an Ethernet domain,
and that the PTP-type stream is the widely known and widely used PTP
packet stream (according to IEEE 1588v2). Each PTP stream comprises only
PTP protocol messages of various types.

[0024] According to a first aspect of the invention, there is provided a
method for timing distribution (e.g. clock distribution) in a
packet-switched network, the method being characterized in that at least
one network node is used as a quasi slave node, located at a path
extending between a master node and a slave node, wherein the method
comprising a step of retrieving timing at the network node by passively
(i.e. non-intrusively) processing packets belonging to a PTP-type packet
stream being exchanged between a master node and a slave node and
conveyed via that network node operating as a quasi slave node.

[0025] According to another embodiment, the packet-switched network
comprises an Ethernet domain, and the PTP-type packet stream is a
PTP-type packet stream which is transmitted in conformity with IEEE
Recommendation 1588v2.

[0026] By yet another embodiment, the slave node is operative to
exchanging PTP-type messages with the master node to enable obtaining the
slave node's timing with reference to the timing of the master node, and
wherein the method comprising:

[0027] selecting an intermediate network node located at the path between
the master and slave nodes, to serve as a quasi slave node;

[0028] ensuring that the intermediate network node has at least partial
functionality of the slave node. This way the selected intermediate node
becomes a quasi (virtual, snooping) slave node;

[0029] at the intermediate network node, passively processing the PTP-type
stream (wherein the PTP-type stream comprises PTP messages exchanged
between the master node and the slave node), and retrieving the timing
for the quasi slave node;

[0030] if the timing retrieved by the quasi slave node does not match one
or more predetermined criteria, generating an alarm to indicate the a
mismatch (e.g. to a monitoring and/or managing entity, such as a network
management system for example).

[0031] In accordance with still another embodiment, the passive processing
of the PTP-type stream at the quasi slave network node comprises
generating internal timestamps for that network node, and associating the
internal timestamps to packets of the PTP-type stream arriving at said
network node. Optionally, this embodiment further comprises refraining
from associating the internal timestamps to packets which have not
arrived at the network node as part of the PTP-type stream.

[0033] According to another embodiment, the predetermined criteria
comprise a specific value of time delay and/or PDV at the point at which
the network node is located.

[0034] In accordance with another embodiment, there is a plurality of
network nodes used as quasi slave nodes, which are located along a path
extending between the master node and the slave node.

[0035] By another embodiment, the PTP-type stream is a PTP packet stream
consisting of PTP packets/messages. It is assumed that a PTP-type packet
steam is established between a master node and a slave node in the
packet-switched network, prior to passively processing the PTP-type
stream by the quasi slave node, and to retrieving the timing for that
quasi slave node, as provided by the present invention.

[0036] The term "passive processing" as used herein (e.g. by implementing
partial functionality of the virtual slave node) may be understood as the
functionality to process the PTP-type stream and the messages exchanged
between the Master node and the Slave node, in order to generate
measurements/timing data there-from. This partial functionality is
passive and does not require/include the ability to generate PTP-type
messages at the virtual slave node itself (say, towards the Master node).

[0037] In general, the term "PTP-type messages" as referred to herein
should be understood to encompass packets associated with timestamps
which are generated by the Master node and the Slave node and exchanged
there-between. To be more specific, PTP messages physically carry an
original timestamp of a Master node, while other timestamps are produced
at a Slave node and logically associated with the PTP messages when they
arrive at/depart from the Slave node.

[0038] The partial functionality of the virtual slave node, i.e. the
processing of the PTP stream arriving at that virtual (quasi) slave node,
comprises generating its local/internal timestamps and associating them
with outgoing PTP packets of the "intercepted" PTP stream, without
generating its own PTP messages.

[0039] The PTP stream processing at the virtual slave node may be
performed for obtaining the timing specific for the particular
intermediate node within the network. The processing may include clock &
phase recovery, measuring packet delay and/or packet delay variation PDV
for the particular intermediate node, and the like. Still, the
functionality of the virtual slave node is passive since the processing
does not include self-generation of messages that are normally generated
by a conventional slave node.

[0040] In another embodiment, the master node and the slave node are both
nodes located in respective access networks interconnected by a network
such as MEN.

[0041] By yet another embodiment, the master node and the slave node are a
central node and a cellular base station (nodes B), respectively, wherein
the Master node is provided with a precise clock (e.g. atomic clock, GPS,
etc.) whereas the Slave node obtains its timing with reference to the
Master node.

[0042] According to another aspect there is provided a network node
adapted to operate in a packet-switched network and provided with
hardware and/or software means enabling the network node to operate as a
quasi slave node, adapted to intercept and passively process packets
belonging to a PTP-type packet stream being exchanged between a master
node and a slave node and conveyed via that network node operating as an
intermediate node, and retrieve timing information therefrom.

[0043] In accordance with another embodiment, the packet-switched network
comprises an Ethernet domain and said PTP-type packet stream is a
PTP-type packet stream which is transmitted in conformity with IEEE
Recommendation 1588v2.

[0044] By yet another embodiment, the network node comprises a processing
unit adapted to process the packets belonging to a PTP-type packet
stream, the processing unit is adapted to process the arriving packets
and retrieve timing for the network node, and in case the timing
retrieved by the network node does not match one or more predetermined
criteria, the processing unit is further adapted to generate an
indication related to this mismatch.

[0045] According to still another embodiment, the network node is adapted
to operate selectively in a conventional slave mode and/or in a quasi
slave mode.

[0046] In accordance with another embodiment, the network node is adapted
to be installed as an edge node located at the border of an Ethernet
network domain and of an access network, thereby enabling carrying out
diagnostics of problems occurring in the Ethernet network domain and/or
in the access network.

[0047] Still, virtual nodes may be installed at more than a single point
along the path extending between the Master node and the Slave node,
thereby enabling the operator to check the timing distribution over the
Ethernet network and/or the access network(s).

[0048] It should be noted that processing of the PTP data packet stream at
the virtual slave node is typically performed without terminating the
data stream (e.g., by processing copies of its packets), which means that
the fact that such a virtual slave is installed somewhere along the path
is not necessarily recognized by the Master node.

[0049] The network management system may be, for example, NMS of the
Ethernet network domain. The Ethernet network domain may be a Metro
Ethernet Network MEN.

[0050] The predetermined criteria may, for example, comprise a specific
value of time delay and/or of PDV at a network point being served by the
intermediate node. For example, when the time delay variation is too high
(i.e., the timing at the point of interest cannot be set to accommodate
such a delay), an alarm may be sent to the management system in order to
indicate the existence of a problem in that section of the network,
thereby providing the possibility to initiate maintenance operations
thereat.

[0051] Preferably, the passive processing is similar to a conventional
processing carried out by the slave node, with the exception that at the
intermediate node (where passive processing is carried out) no PTP-type
packets are generated.

[0052] The quasi slave node may perform various measurements that can be
used for monitoring and managing the packet-switched network. Preferably,
it may be adapted to calculate timing at its location within the network,
and consequently to support/perform timing distribution within the
packet-switched network.

[0053] In other words, the proposed network node, adapted to operate as an
intermediate node, is provided with a processing unit, PU, for processing
a PTP-type packet stream being conveyed along a path extending between a
master node and a slave node, wherein the proposed network node being
capable of processing the PTP-type packet stream of messages exchanged
between the master node and the slave node, each packet of the PTP-type
packet stream carries an original master's timestamp, to obtain the slave
node's timing with reference to the master node's timing, wherein the PU
is adapted to:

[0054] passively process packets belonging to the PTP-type packet stream
being exchanged between the master node and the slave node, by utilizing
original and local timestamps of the packets being passively processed,
and to calculate timing for the proposed network node (operative as an
intermediate node); and

[0055] in case the timing as calculated at the proposed network node does
not meet one or more predetermined criteria, the PU is adapted to
generate an alarm to a monitoring and/or managing entity (for example, to
a network management system).

[0057] According to another embodiment, the network node is provided with
a dual functionality thereby it may be selectively operative either as a
conventional slave node or as a snooping slave node, as may be required
by the network operator.

[0058] By yet another embodiment, the network node is operative to
simultaneously process different PTP-type packet stream.

[0059] According to another aspect, there is provided a computer readable
medium storing a computer program for performing a set of instructions to
be executed by one or more computer processors, the computer program is
adapted to perform the method described hereinabove.

[0060] The computer readable medium is preferably designed to be installed
at a network node which could then serve as an intermediate network node
(a virtual slave node), and/or a conventional slave node.

[0061] According to still another embodiment the computer readable medium
is adapted to enable a network node at which it has been installed, to
operate in two modes, i.e. to operate on one PTP-type packet stream as a
conventional slave node, and to operate on another PTP-type packet stream
as a quasi slave.

[0062] By yet another embodiment, the network node may operate in the two
modes simultaneously when operating on two on more PTP-type packet
streams arriving simultaneously thereat.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] For a more complete understanding of the present invention,
reference is made to the following detailed description taken in
conjunction with the accompanying drawings wherein:

[0064] FIG. 1 (prior art) shows a known way of timing
distribution/measuring packet delays in an Ethernet packet-switched
network;

[0065] FIG. 2 (prior art) shows a conventional way of timing distribution
in combined networks comprising an Ethernet domain, by using a PTP
protocol;

[0066] FIG. 3 (prior art) illustrates a schematic view of PTP messages
being exchanged between a conventional master node and a conventional
slave node, for exchanging time stamps and for calculating delay;

[0068] FIG. 5 illustrates a schematic view of the proposed method for
timing distribution to any required point within a packet-switched
network, without providing additional conventional slave nodes;

[0069] FIG. 6 illustrates schematically exchange of time stamped messages
between conventional master and slave nodes that can be intercepted and
utilized by a virtual slave node;

[0070] FIG. 7 is a schematic block diagram of the proposed virtual slave
node; and

[0071] FIG. 8 is another schematic block diagram demonstrating
functionalities of a network node which may serve as a conventional
and/or as a virtual slave node.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] In this disclosure, the term "comprising" is intended to have an
open-ended meaning so that when a first element is stated as comprising a
second element, the first element may also include one or more other
elements that are not necessarily identified or described herein, or
recited in the claims.

[0073] In the following description, for the purposes of explanation,
numerous specific details are set forth in order to provide a thorough
understanding of the present invention. It will be apparent, however,
that the present invention may be practiced without these specific
details. In other instances, well-known structures and devices are shown
in block diagram form in order to avoid unnecessarily obscuring the
present invention.

[0074] FIGS. 1 and 2 have been described in the background section of the
description.

[0075] FIG. 3 shows a master node's time line 30 and a conventional slave
node's time line 40. The description of the following example is
compatible with the PTP as disclosed in IEEE 1588v2. Let us assume that
the master node sends a PTP data stream (not shown) to the slave node,
and provides the PTP stream with an inbound message. For example, the
message Sync is provided with time stamp T1, being the master node's
reference time. There is also an optional message Follow Up sent from the
master node to the slave node. Let us assume that the message Sync is
received at the slave node at time T2. Upon receiving the Sync message or
the optional Follow Up message, the slave node sends to the master node a
Delay request message and marks it with the time stamp T3. In response,
the master node replies by sending the Delay response message marked with
its own corresponding time stamp T4. Since the slave node has now all the
time stamps required, it may calculate the mean path delay MPD (half
round trip delay) using the equation provided by the PTP protocol.

[0077] The PU 50 receives all PTP data packets (messages) provided with
time stamps that arrived at RX port 51 and generates and forwards PTP
data packets with time stamps towards port 53. Hardware block 58
generates and logically associates local timestamps with incoming PTP
packets/messages, while hardware block 59 generates and associates local
timestamps with outgoing PTP packets/messages. Blocks 58 and 59 inform
the PU 50 about the generated local timestamps.

[0078] PU 50 comprises block 52 operative to process the Sync (and
optionally the Follow_Up) packets received from master node(s), wherein
each such a packet physically carries the first (original that was
generated at the master node) time stamp T1, and arrives to the slave
node at time T2 (see FIG. 3). Both time stamps T1 and T2 are registered
at block 52. Block 54 of PU 50 processes Delay_response packets received
from the master node at port 51, and retrieves timestamp T4. Block 56 of
PU 50 is a generator of a Delay request, which is to be sent from
transmission port 53 of the slave node to the master node, and is
associated with time stamp T3. Time stamps T1, T2, T3 and T4 are then
processed by PU 50, e.g. in logical block 55 to establish the mean
(average) path delay MPD, and simultaneously to calculate and set the
timing of the slave node (using the same timestamps). In case the
required timing cannot be reached, for example due to a great delay
variation PDV between the master and the slave, PU 50 may issue an alarm.
The timing and the alarm may be forwarded to a network managing entity,
for example to NMS of the related network.

[0079] FIG. 5 illustrates schematically one embodiment of the solution
provided by the present invention for timing distribution in a
packet-switched network, such as a metro Ethernet network MEN 10
interconnected with access packet-switched networks comprising the master
node 16 and the conventional slave node 20.

[0080] A PTP protocol (preferably IEEE 1588 v2) is used for generating
various data packet flows in the network. Let us assume for example that
one of these data flows (marked 11) has already been previously
established to exchange communications between a master PTP node 16
associated with the peer PE node 12, and any node (say, node 20 being a
PTP slave node) associated with PE-C node 15 (PE-C is a peer edge node
associated with a customer, or customer faced peer). Now, let us assume
that the service provider of the MEN network 10 is interested in
obtaining information that relates to the performance of the network
(say, delay, PDV, and/or timing at that point) at a border point where
node 15 is installed. Such information may be required, for example, for
the purposes of diagnostics and/or for locating problematic segments
along service path 11.

[0081] If one were to follow the conventional way of timing distribution,
the border node 15 would have provided such information only if it
becomes an additional conventional slave node. Such a solution could be
unacceptable for master node 16 since it would require establishing an
additional PTP data packet stream and providing the border node 15 with
full slave functionality.

[0082] The PTP functionality required for performing the measurement has
usually been divided between the master and the slave. The slave
functions were to be implemented by the customer premises equipment (CPE)
of an access node (e.g. LTE eNodeB of 4G cellular networks). For such a
case, an additional PTP stream would have been required for each
additional slave node.

[0083] However, due to internal resources limitations of the PTP master
node 16, the latter might be unable to manage both the conventional PTP
slave node 20 and an additional "slave-to-be" node 15.

[0084] The solution proposed by the present invention overcomes such a
drawback. According to this solution the functionality of the PTP slave
node is partially provided by an intermediate node located at a path of
an already existing PTP data stream in a Ethernet network. Using this
existing PTP stream, allows according to the solution provided, creating
a passive "snooping/quasi slave" instead of creating a conventional
active slave.

[0085] Thus, according to an embodiment of the invention, PE-C node (node
15) may be converted into a snooping slave node having partial slave
functionality, so that node 15 will be able to process information
contained within the same PTP stream 11 (i.e., to process the PTP
messages passing through node 15).

[0086] Parameters that are calculated by virtual/snooping node 15 (for
example, the timing of the node) may be reported to a Network Management
System 19. If delay/PDV in the path 11 is higher than a predetermined
threshold, the node 15 may forward an alarm to the NMS, for taking
further decisions by the latter. Other alarms are also possible (for
example, slave leaving its "Locked" state, e.g. due to the growing PDV).

[0087] The snooping slave 15 is preferably operative in a continuous mode,
using an existing master-slave PTP stream for measurement of delays and
PDV, and/or for calculating the timing of the snooping slave (e.g. for
clock+phase recovery).

[0088] The snooping slave preferably does not terminate the packet stream,
but just monitors it. Actually, the proposed method may be considered as
a method to piggyback existing packet flows (e.g. packet streams that
were previously created by a customer), instead of creating its own
packet stream for the purpose of various measurements/timing
distribution.

[0089] As will be appreciated by those skilled in the art, more than one
snooping slaves (for example, node 17) may be provided in the network on
one and the same path of any existing PTP packet stream. Though only one
existing PTP stream is shown in FIG. 5 (between master 16 and slave 20),
there may exist other PTP data streams, say between master node 16 and
other access nodes (not shown) being conventional slave nodes.

[0090] It should be noted that according to this embodiment the master
node does not perceive the proposed snooping slave node as a conventional
slave node, since the latter is passive and thus unseen by/hidden from
the master node. Therefore, the solution provided by the present
invention allows to overcome the prior art limitation on the number of
slave nodes that can be used for a single PTP packet stream.

[0091] It is further proposed by another embodiment of the invention that
a network node may be provided with a dual functionality, i.e. to be
operative either in a mode of a conventional slave node or in a mode of a
snooping Slave node, or both (for different PTP packet stream).

[0092] As would be appreciated by those skilled in the art, the solution
provided by the present invention offers a number of advantages over the
prior art solutions. According to the present invention solution one may
obtain results of PDV measurements for the desired protocol (PTP-type),
and carry out measurements for any number of snooping slave nodes from
the same master node, while keeping the master node (16) itself unaware
of the snooping nodes (15, . . . ) and at the same time to operate
conventionally with the same, unchanged number of real slave nodes (20, .
. . ) that the master slave had been connected to.

[0093] It should also be noted that the above-described quasi slave nodes
may be located at one or both of the access packet-switched networks
shown in this figure.

[0094] FIG. 6 illustrates a time line 35 of the proposed virtual slave
node, located at the network extending between the master node (with its
time line 30) and the conventional Slave node (with its time line 40). If
one compares FIG. 6 with FIG. 3, one may notice that the virtual slave
node does not generate any PTP messages, but just "intercepts"
(non-intrusively) the ones being exchanged between the master and the
slave nodes. Based on the "intercepted" messages carrying original
timestamps T1 of the master slave, the virtual slave node generates its
own, local timestamps T2', T3' and T4' (e.g. by using its internal HW
block shown in FIG. 7). Time stamps T2' and T4' are generated at the
input of the snooping node, whereas timestamp T3'--at its output. The
locally generated timestamps are provided to the processing unit of the
slave node, by logically associating them with the intercepted PTP
messages.

[0095] One should keep in mind that the original time stamp T1 remains the
same since it is the absolute master node's timestamp. Using the original
and the generated local timestamps, the virtual slave node is able to
calculate its mean path delay MPD and its timing according to the same
formula as presented with reference to FIG. 3.

[0096] FIG. 7 illustrates schematically a processing unit PU 60 which may
be provided in a snooping/virtual/quasi slave node. PU 60 differs from PU
50 of the conventional slave node (see FIG. 4) in that there is no
generation of PTP messages and therefore no transmission thereof. The
quasi slave node transparently passes the PTP packets there-through,
though performs its own monitoring and processing of these packets. PU 60
illustrates only this internal processing. PU 60, via a receiving port
(RX) 61, receives all PTP messages/packets intercepted by the virtual
slave node, and associates them with local/internal timestamps generated
at the hardware block 67. Different messages are processed by different
respective blocks 62, 64 and 63 and then by a logical block 65, where the
latter is responsible for processing internal timestamps and for clock &
phase recovery. Using the local timestamps, the processor 60/block 65 may
calculate delay, PDV, obtain its own timing and/or generate alarms.

[0097] FIG. 8 illustrates schematically a block diagram of a network node
designed to have a dual slave functionality, so as to operate according
to at least one selected mode at any given time. In the node, any ingress
PTP packet undergoes classification (block 70) with logical time
stamping, than replication (block 72) in order to produce a copy for
processing. One copy is normally forwarded (block 74) to the next node
along path 76 or processed as may be required to obtain the functionality
of a conventional slave, if the node is operative in that mode (block
50). If a snooping slave mode is selected, the other copy of the received
PTP packet is sent to block 60 (the logical stamping has already been
provided by block 70). When operating by this mode, the normal PTP
forwarding by block 74 remains, to allow transparent passing of the PTP
stream through the node.

[0098] In case the two branches of the node serve different PTP streams,
they may be utilized simultaneously in both modes, i.e. this node
operates as a conventional slave node and as a quasi slave node for two
respective paths.

[0099] Though the invention has been described with reference to a number
of specific examples, it should be appreciated that other versions of the
method and other embodiments of the virtual Slave node could be proposed,
which are to be understood as part of the invention as far as defined by
the claims which follow.